Pure mitochondria, isolated from Euglena gracilis, will be used as a source of mitochondrial ribosomes, polyribosomes and messenger RNAs. These preparations will be used to determine (1) the genetic origin by DNA-RNA hybridization of those messenger RNAs translated on mitochondrial ribosomes and (2) the biochemical basis for the specific requirement of mitochondrial ribosomes for homologous ribosomal proteins during the process of protein biosynthesis. High yields of ribosomes, polyribosomes and mRNAs are readily obtained from Euglena. Thus, the experiments proposed above are both feasible and definitive since, with these high yields, there is no necessity to rely on inhibitors of mitochondria or cytoplasm or the nucleus to obtain data, as is the case with other cellular systems.